A disappointed Lewis Hamilton admitted Mercedes were "a long way off" the pace and struggling to close the gap after he finished eighth in Friday's second practice at the season-opening Bahrain Grand Prix.
The seven-time world champion, who is in the final year of his current contract, said he had half-expected to discover a performance deficit, but not one as alarming as revealed on Friday.
"I'm trying everything I can, but it is what it is and we've just go to try and work at it. We kind of knew that a little from the test, but it's a big gap," he said.
"We're not strong on single lap pace or race pace. I'd say they're very similar. We're a second a lap behind Red Bull so we've got a lot of work to do."
Hamilton added that he had expected Mercedes to be closer to the front.
"We're just on the wrong track so we've got to continue to graft away and find a way to get on the right track," he added.
"Right now we're a long way from the guys in front. Do I believe we can close the gap? Yes, I do, but I think it's quite hard with the concept we have."
Fernando Alonso set the best time for Aston Martin ahead of Red Bull's defending double world champion Max Verstappen.
Alonso, 41, and driving with the enthusiasm of a man half his age, clocked a best lap in one minute and 30.907 seconds to outpace the Dutchman by 0.169 seconds.
Sergio Perez was third in the second Red Bull, just 0.002 seconds adrift of his teammate, but six-tenths clear of Ferrari's Charles Leclerc, ahead of Nico Hulkenberg of Haas and Lance Stroll in the second Aston Martin.
Pierre Gasly was seventh in his first day with the Alpine team ahead of Hamilton whose Mercedes teammate George Russell was 13th.
"I think I got the car to the best place I can get it to, set-up wise," added Hamilton.
"We'll continue to tweak little bits here and there but it's going to be small bits here and there, which is milliseconds.
"It's not going to be the closing of the gap of a second. We'll just keep our head down tonight, go through the data, continue to work and try to progress tomorrow."